Nearly 22 percent of criminal cases tried by the office of Maricopa County Attorney Andrew Thomas last year ended with the defendant being found "not guilty," according to statistics released by Thomas' office.

Thomas released the numbers in response to a press release from his opponent, Tim Nelson. Nelson had initially claimed that the Maricopa County Attorney's Office suffered a 29 percent acquittal rate in 2007 -- but had to back away from that claim after it turned out the data set used by his campaign was incomplete.

However, the acquittal rates released by Thomas do confirm Nelson's central contention: More defendants are being found not guilty at trial.

Nelson, former general counsel for Governor Janet Napolitano, is challenging Thomas for his seat this November. For months, Nelson's campaign claimed that acquittal rates under Thomas have increased from those of his predecessor, Rick Romley, citing statistics from the public defender's office. Thomas' spokesman repeatedly disputed that.

The statistics released by Nelson's campaign last week, followed by Thomas' release Friday, provide a more complete picture than was previously available. The public defender statistics cover only trials handled by that office. But, according to Thomas' spokesman, Mike Anthony Scerbo, the statistics released by Thomas include every case that's gone to trial. And the stats suggest that Thomas' office has, in fact, been winning fewer cases than the office did under Romley.

So here's a little primer on the two sets of numbers. Nelson's numbers, obtained via a public records request from the Maricopa County Superior Court, show the outcome of every case that's resolved within six months of its filing. Under those stats, in Romley's last 18 months in office, 16.2 percent of trials ended with a "not guilty" verdict. For Thomas, that rate has been strikingly higher -- and steadily increasing:

The numbers released by Thomas are apparently more complete, though the office only released a four-page set of charts, not the raw data. They show that, under Romley, 16 percent of trials ended with the defendant being found not guilty. Under Thomas, that number has climbed to 20 percent.

"This matches what Tim has been hearing from lawyers active before the courts, and other people in the justice system," says Josh Kilroy, Nelson's spokesman. "Acquittal rates have been on the rise for the last four years."

The data could be a blow to Thomas' claims of being tough on crime. After all, what good does it do to throw the book at someone if you can't manage to get a conviction in trial?

The town has been abuzz with all the high-profile cases that have gone down in the flames, from the laughable charge that Tom Lovejoy supposedly intentionally killed his police dog to the ACLU's legal director being found not guilty for trespassing. This suggests that many less prominent cases have suffered the same fate.

I'll be writing more about this issue for my column this week, so please make sure you take a look.